In the past few years there have been significant developments in the use and disposal of waste products from combustion equipment such as pulverized coal burning power generating stations. In particular, and as is set forth in U.S. Pat. No. 3,785,840 to Minnick et al, highly significant commercial processes and compositions have now been developed which can dispose of the fly ash produced in this equipment. Furthermore, and also in connection with these types of plants, a sludge is produced in the wet scrubbing of stack gases to remove sulfur oxide from the sulfur oxide-containing gases produced therein. The Minnick et al process permits one to simultaneously dispose of this waste material. Furthermore, these waste products have now been utilized in a manner so as to produce hardenable masses or cementitious compositions which can be utilized safely and efficiently for landfill purposes and the like.
The commercialization of these processes has primarily been carried out by Conversion Systems, Inc. and its subsidiary IU Technology Corporation. These processes were initially developed in view of the considerable interest which had arisen in protecting the environment by removing pollutants from various industrial sources, and particularly from the flue gases produced in combustion equipment, such as pulverized coal burning power generating stations.
The invention which is set forth in the above-noted U.S. Pat. No. 3,785,840 is thus of extreme commercial significance because it permits the disposal of not only the finely divided particulate material known as fly ash, but in addition, it permits the simultaneous disposal of the sludge which is produced in the wet scrubbing of sulfur oxide-containing stack gases by means of alkaline earth metal hydrates or carbonates. In this manner, not only is disposal of both of these waste products achieved, but it is achieved in the form of safe and useful landfill composition, which has been highly acceptable from an environmental point of view.
This prior invention has primarily been based upon the discovery that the specific combination of this sludge material, the fly ash, and sufficient alkaline earth metal oxide or hydroxide can result in a hardenable mass or cementitious composition. These materials can also be used for road base compositions, and other useful applications aside from pure landfull applications.
While these applications of the invention set forth in U.S. Pat. No. 3,785,840 have thus been of considerable significance from a commercial viewpoint, there has now developed an additional need for such products having improved physical characteristics, and most significantly improved permeability. That is, particularly where these compositions are to be used as landfill, the rainfall permeating such a landfill mass can significantly affect the ground water contamination levels thereof, and a low quantity of water permeating the landfill mass is therefore most desirable.
In addition, and particularly in those circumstances where these compositions are to be used in road bases or other load supporting structures the overall strength of these compositions also becomes a significant factor.
The hardenable cemetitious compositions produced by the method disclosed in the Minnick et al patent generally have a permeability coefficient of not less than about 2.times.10.sup.-6 cm/sec, and in fact in the actual application of that invention to pulverized coal burning power generating stations and the like, typical guarantees for the product produced thereby usually include a guaranteed permeability coefficient of the cured material of, at best 5.times.10.sup.-6 cm/sec. Furthermore, these compositions are generally guaranteed to have a compressive strength of no greater than about 25 psi.
With respect to various actual power company installations, among the lower permeability coefficients which have been measured, generally in connection with long term curing, is a coefficient of 3.13.times.10.sup.-7 cm/sec in connection with the Lakeland Department of Electric and Water Utilities installation for a 28 month period.
In a report by the Electric Power Research Institute, dated September 1982, entited "Landfill Disposal of Limestone Dual Alkali Flue Gas Desulfurization Waste", Arthur D. Little, Inc. analyzed the materials produced in a dual alkali system, namely one in which an alkali metal compound, namely a sodium compound, is used in the actual scrubbing process, while an alkaline earth material, namely calcium (lime), is employed outside of the alkali metal compound "loop", that is, so as to replace the alkali metal in the final product produced therein. The overall chemistry in this system is thus quite different from that in a conventional alkaline earth metal compound scrubbing system. In any event, this rather extensive study reached a number of conclusions, including the fact that, with higher lime contents longer times were required to develope maximum cured strength, and that for mixes with from 1 to 3% lime, a 28 day curing period was adequate, while this was not the case with higher lime contents. It was also determined that the compressive strength of the samples tested varied depending upon alterations in the lime, fly ash and solids content of the product. Thus, with solid contents of 56% and lime contents of 53%, by raising the ratio of the fly ash to the flue gas desulfurization filter cake up to about 1/1, the unconfined compressive strength was increased to about 25 kg/cm.sup.2 for air dried samples. With respect to permeability coefficients, these tests demonstrated that the permeability coefficient for these compositions generally decreased with curing time, and that for lime contents of from about 3 to 5%, the permeability coefficient was found to decrease approximately 100 fold after curing. In particular, tests demonstrated permeability coefficients of 7.4.times.10.sup.-7 cm/sec after 28 days of curing. The specific data underlying the conclusions in this report include individual samples which exhibited permeability coefficients of as low at 2.1.times.10.sup.-8 cm/sec after 28 days of curing. However, not only did these tests relate exclusively to dual alkali systems, but the results obtained generally showed much higher permeability coefficients, and as indicated, the authors themselves indicated that ". . . coefficients of permeability for intact cured Plant Scholz stabilized FGD sludge of 10.sup.-5 to 10.sup.-6 cm/sec are considered representative". Indeed, the actual data upon which these estimations were apparently based did not even approach permeability coefficients which were this low.
In another report by the Electric Power Research Institute, in this case prepared by Michael Baker, Jr., Inc. and entitled "FGD By-Product Disposal Manual, Third Edition", dated January of 1983, it is generally stated that the use of this material as a liner for an evaporation pond at the Four Corner Station at the Arizona Public Service Company in 1974 resulted in the permeability of the mass being reported to be between 10.sup.-6 to 10.sup.-8 cm/sec after curing. The report also indicates that "possibly the liner could be made from the fixated waste materials if adequate quantities having permeabilities less than 10.sup.-7 cm/sec could be produced by properly modifying the treatment procedures". This report also indicates that these materials tested in unconfined compression exhibited strengths of several thousand pounds per square foot.
In a report entitled, "Geotechnical Evaluation of Stabilized FGD Sludge Disposal", by Ruggiano and Poulson, presented at the Second Conference on Air Quality Management in the Electric Power Industry on Jan. 24, 1980, further such materials were extensively tested. In particular, both the unconfined compressive strength and the permeability of actual samples contained in various sites, including that of the Columbus and Southern Ohio Electric Company's Conesville station site, were carried out. Increased unconfined compressive strength and decreased permeability were demonstrated with increased aging or curing of the samples involved. Specifically, unconfined compressive strengths for less than two months of aging were generally below 150 or 200 psi, although figures in excess of 250 psi were uncovered for long term samples. As for permeability, for samples aged less than two months, permeability coefficients of no less than about 5.times.10.sup.-7 cm/sec were obtained. In particular, in one test the result of 2.9.times.10.sup.-8 cm/sec were obtained for a long term sample, and unconfined compressive strengths of as high as 382 psi were uncovered for some such samples.
In the previous methods which have produced these compositions, the ratio of the amount of fly ash to the flue gas desulfurization (FGD) filter cake is generally solely a function of the particular plant in question, i.e. by the amount of such waste products which are actually produced therein. Thus, it has generally been known that in such commercial operations the ratio of fly ash to filter cake generally ranges from about 0.5/1 to about 3/1. It has also been recognized that these ratios can influence the ultimate permeability characteristics of the product produced thereby.
In connection with the mixing of these ingredients, it has generally been a significant concern in the past to limit the amount of such mixing, since overmixing has been known to result in the production of a thixotropic substance, which is difficult to handle, and which has therefore generally been scrupulously avoided. In the article "Mixing to Improve Handling Characteristics of Thixotropic Sludges", by D'Alonzo, presented at the 103rd Winter Annual Meeting ASME, November of 1982, it is specifically stated that undermixing does not provide adequate dispersion, but that overmixing deteriorates handleability of the product. It is further stated therein that "overmixing can trigger the thixotropic behavior of the original sludge so that the material becomes viscous and more difficult to handle".
It is therefore a primary object of the present invention to provide a method for producing such hardenable masses which have improved physical properties, including permeability coefficients and compressive strengths.